Plug door

A plug door is a door designed to hold higher pressure on the side it opens to - the shape of the plug door (typically like a slice of a wedge base) prevents it from being opened when there's a difference in pressure across the door.

Non-plug doors rely on the strength of the locking mechanism to keep the door shut, whereas a well-designed plug door relies only on the strength of the wall around it and the material the door itself is made from.

Uses

On aircraft

The plug door is often seen on aircraft. Due to the air pressure within the aircraft cabin being higher than that of the surrounding atmosphere, the door seals itself closed as the aircraft climbs and the pressure differential increases. This prevents the opening of a plug door on board a pressurized aircraft.

However, in the event of a decompression, with there no longer being a pressure differential, the doors may be opened, and as such most airlines operating procedures require cabin crew to keep passengers away from the doors until the aircraft has safely landed.

On buses

Some buses had also been spotted with plug doors. This had existed on some Australian and on some of the buses in Singapore and Hong Kong. Main bus operators in Singapore such as SBS Transit have used plug doors on the buses with the models Volvo B10M (SBS2754X), Volvo B10TL Super Olympian (SBS 9800A to SBS9849K, SBS9888Y and SBS9889U), Scania L94UB (SBS2888T), Dennis Trident 3 (SBS9671E to SBS9690A), Volvo B10BLE CNG (SBS2988M to SBS2999G), Volvo B9TL (SBS7300P to SBS7499A), Scania K230UB (SBS8033D to SBS8532E, SBS8433H to SBS8477G and SBS8888D), Volvo B7RLE (SBS8030L) and MAN 18.240 HOCL-NL (SBS8031J).

On spacecraft

This style of door was used early in the US Space Program on the "Block I" Apollo Command Module; however, after the Apollo 1 fire that killed the three astronauts during a ground test, NASA decided to return to outward opening doors, using plug doors only for the CM docking hatch, and the two hatches on the Apollo Lunar Module (as a fire in the low-pressure atmosphere was easily extinguishable, the Apollo 1 fire occurred in a high-pressure pure oxygen atmosphere). Currently, plug doors are used only on the outer hatch doors on the Space Shuttle and International Space Station as well as on the hatch between the Orbital Module and Descent Module on the Russian Soyuz spacecraft.